The present invention relates to an electrophotographic photoreceptor which has an electroconductive substrate and a photosensitive layer of amorphous selenium based material formed on the substrate, said photoreceptor being designed for the reversal development process.
The electrophotographic photoreceptor (referred to hereinafter as "photoreceptor") is a device comprising an electroconductive substrate of an aluminum alloy or the like and a photosensitive layer of a photoconductive material formed on the substrate. Where the photoreceptor of this type is used for a printer, it is usually run by the reversal development process which employs the positive corona discharge for charging and the negative corona discharge for transfer. This process consists of the following steps: First, the photosensitive layer is positively charged by positive corona discharge in the dark. The charged surface is then irradiated (exposed) with laser light corresponding to an image. The irradiated parts decrease in potential, giving rise to the highlight potential, and the unirradiated parts retain the charge, giving rise to the shadow potential. In this way, an electrical image or an electrostatic latent image is formed. Then, a positively charged toner is deposited on the highlight parts having a lower potential in the developer. The toner is transferred to paper which is subjected to negative corona discharge on its reverse side. The transferred toner is fixed thermally or chemically, and the toner which has remained untransferred on the photosensitive layer of the photoreceptor is removed by a fur brush or blade in the cleaning step. The remaining charge is then removed by means of light or alternating current, and the cycle is repeated from step one.
A photoreceptor of this type is directly exposed, during transfer, to negative corona discharge through the perforation of continuous form paper or the intervals of single sheet feed. As a result, the photoreceptor itself becomes negatively charged. If this negative charge is excessively high, the photoreceptor becomes less receptive to positive charging, causing the highlight potential and shadow potential to decrease. This results in a difference in printing density which occurs at the time of transfer between the part covered by paper and the part not covered by paper, thus adversely affecting the printing quality.
In the case of a photoreceptor made of an amorphous selenium-based material, the negative charge which has occurred at the perforations or intervals of paper by negative corona discharge at the time of transfer does not decay easily on account of the low mobility of electrons. It is believed that the decay of negative charge is by the mechanism in which holes are injected from the substrate and move to the surface by the action of an electric field to cancel out the negative charge on the surface.
There is known a photoreceptor which has a single photosensitive layer or a double-layered structure, consisting of a carrier transport layer adjacent to the substrate and a carrier generation layer. Such a photoreceptor is poor in negative charging characteristics because there exists an insulating oxide film in the interface between the substrate and the photosensitive layer or carrier transport layer which lowers the injection of holes; and because of the low mobility of holes in the carrier transport layer.
It is an object of the present invention to solve the above-mentioned problems and improve the efficiency of hole injection between the carrier transport layer and carrier generation layer, thereby providing an electrophotographic photoreceptor having good negative charging characteristics.